Infrared spectra of commercial preparations of the opioid agonist dynorphin A-(1–13) contain an absorption band at 1673 cm⁻¹. While this band can be, and previously has been, interpreted as indicative of a β-sheet structure in dynorphin A-(1–13), we have now established that the band does not originate from the peptide per se, but that it arises from the antisymmetric COO⁻ stretching vibration of the counterion trifluoroacetate. After repurification, the IR spectrum of dynorphin A-(1–13) is devoid of the 1673 cm⁻¹ band and is highly characteristic of an unordered conformation of this peptide in aqueous solution. Trifluoroacetate and other organic counterions are likely to be present in commercial preparations of other synthetic peptides, and one should be careful with the interpretation of their IR spectra in the region of the conformation-sensitive amide bands.